This invention relates to a shift control system of an automatic transmission, and more particularly to an automatic transmission wherein the shift control system includes first and a second transmission sections capable of automatically switching gear stages separately of one another. The first and second transmission sections are shifted simultaneously or alternately to achieve multi-stage shifts.
Along with a marked increase in the use of automatic transmissions to motor vehicles to improve fuel consumption, there has been introduced adopted transmissions wherein a socalled overdrive device, in which a transmission gear ratio is less than 1. The device is connected in series to an output shaft of a first transmission section capable of automatically shifting gear stages as a function of vehicle speed, throttle opening and the like, as a second transmission section.
Furthermore, such a transmission is also known, wherein the second transmission section is actively tuned to shifts of the first transmission section, such that when the first transmission section and the second transmission section are simultaneously or alternately shifted, for example when the shift control shown in FIG. 2 is performed, multi-stage shifts of six forward gear stages can be achieved.
The above-described arrangement makes it possible for existing automatic transmissions to be utilized as the first transmission section, whereby changes in design are minimized, so that multi-stage shifts can be achieved. As a result, many advantages are achieved such as improving the rate of fuel consumption, shift shocks are reduced and load is relieved from a frictionally engaging device by allowing multi-stage shifts.
However, in the automatic transmission wherein the first and the second transmission sections are shifted simultaneously or alternately to thereby achieve the multi-stage shifts, cases occur in which new gear stages are achieved by shifting the first and the second transmission sections in reverse directions as during an up shift from 2nd gear to 3rd gear, an up shift from 4th gear to 5th gear or down shifts reversing the aforesaid up shifts. At this time, such disadvantages have been presented that when the respective shifts are controlled separately of one another, an increase in shift shock is not avoidable. Moreover, while an up shift is in operation, the shift is started from a down shift, an up shift is performed immediately after a down shift or a down shift is performed immediately after an up shift, the shift characteristics cause undesirable driving feelings.
And, when the completion of a shift in the second transmission section is earlier than the completion of a shift in the first transmission section, an abrupt increase in output shaft torque is produced in a remaining portion of the inertia phase in the first transmission section, thus increasing shift shock.
Furthermore, when shift timings of both transmission sections are only controlled by timers, oil-pressure levels of frictionally engaging devices or the like, it is extremely difficult to synchronize the controls of the aforesaid delicate starting and ending timings with intended timings. More specifically, shift starting times and inertia phase (during real shifting) of shift are delicately dispersed due to variations in parts of the automatic transmission and in use conditions, whereby errors in shift control timings due to these variations cannot be neglected.
Furthermore, the a problem has also been raised that the time from a shift start command to the start of an inertia phase varies depending on the temperature of oil in the automatic transmission, a shift interval (the elapsed time from the preceding shift) and the like, such that it is difficult to make the shift completions of both transmission sections coincide with each other.